In multiple areas the use of RFID systems for the identification of arbitrary objects has increased during the last years. The term RFID stands for radio-frequency-identification and designates an identification by means of radio waves. An RFID system is always comprised of two components: an evaluation unit, which can be formed as read and/or write unit; and a transponder, which carries the data used for the identification.
As a result of simple handling and a robust formation, transponders have an insignificant susceptibility and, therefore, offer a series of interesting possibilities of application. Thus, transponders are already used today in distinctive areas, as e.g. in access control systems, embedded in access tickets, wrist-watches, key tie-on labels or corporate identification cards, in monitoring systems, embedded in paper or plastic labels or also in credit instrument systems, embedded in contactless chip cards.
The at present most fabricated transponders comprise a coupling element on a substrate, which is connected with a small surface chip. The coupling element itself is either formed as conductor loop for a predominantly inductive coupling, or as dipole. In the case that the coupling element is formed as a dipole, the transponder can be used both in a so-called “close-coupling-system” as well as in a “long-range-system”.
Close-coupling-systems are RFID systems with very small ranges, in which the dipole of the transponder makes a pure capacitive signal coupling of an evaluation unit possible, said evaluation unit being located in a small distance of about 0.1 cm to 1 cm from the transponder and comprising likewise suitable electrode surfaces. For signal coupling, both dipoles are positioned parallel to each other, and thus, form plate capacitors, over which the data and/or energy are transmitted.
In long-range-systems, ranges of between 1 m to 10 m distance between transponder and evaluation unit may be achieved. In long-range-systems the dipole is formed as dipole antenna and is operated with very high frequencies, which are in Europe currently located at approximately 2.45 GHz, resp. 5.8 GHz. Therein, a power is irradiated over an evaluation unit, said power being provided to the terminals of the dipole antenna of the transponder as a high frequency voltage and used after rectification as supply of the chip.
In the transponders with dipole that are presently in common use, a chip or a chip module is connected over corresponding contacts with the dipole, in order to be able to communicate with the outer world.
According to this state-of-the-art, the electrical contacting of the chip with the dipole is obtained by placing the chip on a substrate, on which the dipole is formed and thereafter connecting the chip with the dipole.
For achieving this electrical connection between dipole and chip, especially the use of chip modules, the so-called “flip-chip”-method as well as the wire bonding method are generally adopted and will be described in detail in the following.
FIG. 2 explains the principle, of how according to the state-of-the-art an electrical connection between a chip module 1 and a dipole 4 is commonly produced.
A chip module 1 that includes in it's interior a not-shown chip, has contacts 2, which are accessible from the exterior and which are, for instance, connected with the actual contacts of the chip by means of a soldering process, an adhesive process or a wire bonding process. By reference sign 3, contact terminals are designated, which are directly connected with the dipole 4 that is placed on a substrate 6. To finally achieve a contacting between the dipole 4 on the one side and the chip on the other side, the contacts 2 must be conductively connected with the contact terminals 3. Thereto, the module 1 is placed by means of a tool with its side comprising the contacts on the contact terminals 3.
Another possibility is to place in a flip-chip process the chip 5 itself, that means not in the form of a module, but “naked” on the contact terminals 3, as illustrated in FIG. 2a. According to this possibility, the chip 5 must be placed with its “active side”, that means the side that carries the contact pads 7 on the contact terminals 3. Thereto, the chip must be “flipped”, thus requiring a double gripping of the chip 5, e.g. from a carrier tape.
FIG. 3 illustrates the possibility, of how according to the state-of-the-art an electrical connection between a chip 5 and a dipole 4 can be produced by means of a wire bonding process.
To achieve in this case a contacting between the dipole 4 on the one side and the chip 5 on the other side, the chip is placed on the substrate 6 and the contact pads 7 are conductively connected with the contact terminals 3 by means of wire bonding. As can be seen, according to this solution the chip 5 must no more be flipped. However, the wire junctions generated by the wire bonding lead to an increased total mounting height of the transponder and must moreover be protected by a solid protective coating.